In the production of aluminum, there have been numerous proposals for composite materials based on alumina or other refractory oxycompounds and the so-called Refractory Hard Metals.
In this specification, the term "Refractory Hard Metals" or "RHM" designates the borides and carbides of Groups IVB (Ti, Zr, Hf) and VB (V, Nb, Ta) of the periodic table of the elements. The RHMs contemplated for this invention are aluminum-wettable. Also, "RHM" as used herein includes composites based on RHM and which include sufficient RHM at least at their surface to provide permanent wettability to molten aluminum as well as other materials stable in the presence of molten aluminum and wettable by the molten aluminum.
Conventional Hall-Heroult cells for the electrolytic production of aluminum employ a carbon cell bottom which serves to supply current to a deep pool of molten aluminum forming the cathode. The cathodic aluminum is necessarily thick (at least 80-100 mm) because carbon is non-wettable by molten aluminum. Problems arise because of wave motion in the thick aluminum pool caused by the considerable forces generated by the magnetic fields. The use of RHM to alleviate these problems has been contemplated, but so far has not met with success.
U.S. Pat. Nos. 4,650,552 and 4,600,481 described families of composite materials including conductive alumina-aluminum composites having RHM additives to enhance wettability and electrical conductivity. These materials were mainly intended for use as components such as cathodes and cathode current feeders which serve to supply electrical current to the cell. Considerable development work did not, however, lead to a material which combined electrical conductivity, wettability, resistance to molten aluminum and cryolite and other desirable properties.
U.S. Pat. No. 4,560,448 describes a structural component of an aluminum production cell which is in contact with molten aluminum, made of a non-wettable material such as alumina which is rendered wettable by a thin coating of TiB.sub.2. However, to prevent dissolution of this thin (up to 100 micron) coating, the molten aluminum had to be maintained saturated with titanium and boron.
EP-A-0117366 describes the production of a composite refractory cermet material by filtering, e.g. titanium diboride particles in molten aluminum through a porous refractory followed optionally by comminuting into green shapes and firing to form a dense sintered tile. Generally, sintering aids are necessary and sintered bodies are inherently porous leading to attack and grain-boundary corrosion when in contact with molten aluminum.
A dense essentially non-porous titanium diboride-alumina composite material of specific microstructure is described in U.S. Pat. No. 4,647,405. In this microstructure, submicronic boride is distributed in intimate wetting contact with the surfaces of alumina agglomerates. This material shows promise for use in the environment of an aluminum production cell but the manufacture of large components is expensive.
A hard, dense refractory vitrified composition consisting of alumina with aluminum nitride is known from U.S. Pat. 2,480,475. Such material is non-wettable by molten aluminum, and its use is, therefore, limited. Another hard material proposed in French Patent Application 7613195 for abrasive applications is a composite of silicon carbide in fused alumina. This material is also non-wettable by molten aluminum and cannot be used in applications where such wettability is a requirement.
Various electrically-conductive composites composed of an electrically conductive matrix (such as graphite with pitch and other binders) including particles of RHM are also known, e.g., from U.S. Pat. Nos. 3,661,736, 4,376,029, 4,465,581, 4,466,996, as well as from WO 83/04271 and WO 84/02930. These materials are intended mainly as a replacement of the conventional current-carrying carbon lining of aluminum production cells, but so far have not found acceptance.
It has also been proposed to cover the carbon bottom of Hall-Heroult cells with tiles or slabs of RHM such as TiB.sub.2. For example, U.S. Pat. No. 4,231,853 discloses fixing TiB.sub.2 files loosely on pins. However, the cost of TiB.sub.2 tiles is high and attaching the tiles to the bottom involves difficulties.
The aluminum production cell described in U.S. Pat. No. 4,383,910 employs a carbon cell bottom from which TiB.sub.2 tiles project upwardly to form the cathode surface. On the cell bottom, surrounding the tiles, is a layer of solid cryolite/alumina which protects the carbon bottom from attack by molten aluminum. Again, this cell has not proven to be practical.